The present invention relates in general to cryostats and in particular to cryostat apparatus for reducing heat leakage into the interior of a vessel that must be kept at a low temperature, e.g. a vessel containing a superconducting magnet immersed in a liquid cryogen, such as helium which has a boiling point of approximately 4 K. In order to maintain the interior of the vessel at its low temperature and to contain the operating costs of the cryostat by reducing the loss of cryogen due to boil-off, currently available cryostats take elaborate measures to minimize the amount of heat that leaks into the cryostat vessel. This is particularly important with respect to the well structure through which the vessel is accessed from its external environment.
For example, in order to reduce the heat leaked into the vessel by electrical wires that pass through the well structure, it is known to intercept this heat leak by using the cryogen vapor which is formed during the boil-off of the liquid and which is otherwise vented to the external environment. Heat may also be transferred into the vessel by the gaseous conduction of the cryogen vapor in the well, by radiation into the well, and by solid conduction of the well material. It is known to use reflective baffles in order to reduce the amount of heat radiated into the well. Notwithstanding these measures, it is estimated that in currently available cryostats as much as 60% of the total heat leakage into the vessel occurs through the well structure.
Accordingly, it is a principal object of the present invention to substantially reduce the amount of heat that leaks into a cryostat vessel through its associated well structure during the operation of the cryostat.
It is a further object of this invention to reduce the operating costs of a cryostat by reducing the liquid cryogen boil-off rate.